(1) Field of the Invention:
This invention relates to a method for continuous thermal cracking of heavy petroleum oil. More particularly, it relates to a method for continuous thermal cracking of heavy petroleum oil, which comprises charging the heavy petroleum oil into the upper reaction zone of an upright cylindrical reactor, which is divided into upper and lower reaction zones, and discharging the resultant cracked gas and cracked oil vapor and the resulting residual pitch, respectively, from the upper and from the lower reaction zones. This invention also relates to a first embodiment of the method for continuous thermal cracking of heavy petroleum oil in which, upon obtaining a residual pitch the reaction liquid leaving the reactor is introduced into a cooling chamber and a defoaming chamber so as to cool the reaction liquid, thereby terminating the reaction, and to defoam the resultant pitch; the invention also relates to a second embodiment of the above method in which the upright cylindrical reactor is provided with a rotary scraper having blades which are contiguous to their corresponding reactor walls or partition plates.
(2) Description of the Prior Art:
With a view toward making an effective use of heavy petroleum oil, a wide variety of attempts has heretofore been made to obtain gas, cracked light oil and pitch from such heavy petroleum oil by thermal cracking.
It is extremely important for an effective use of heavy petroleum oil, for savings in usable coal and for employing a wide choice of usable coals to utilize pitch produced from heavy petroleum oil as a binder in making coke for steel-making blast furnace or as a substitute for caking coal which is also used to make such coke. Needless to say, it is also very important to improve the yield of light oil to be obtained by cracking.
In order to make pitch obtained through the thermal cracking of heavy petroleum oil suitable in quality for the above applications, it is necessary that such pitch has a composition rich in components insoluble in benzene and lean in components insoluble in quinoline, in other words; a composition rich in .beta.-resin components which have an average degree of carbonization and polycondensation.
A variety of batch and continuous methods for cracking of heavy petroleum oil are known. However, it is difficult to obtain pitch suitable in quality for the above applications in accordance with any such conventional methods. The present applicants have already proposed to conduct the thermal cracking of heavy petroleum oil by causing the heavy petroleum oil to flow upwardly through a reactor equipped with at least one partition plate defining an opening at the center thereof while controlling its residence time within 1-10 hours (Japanese Patent Application No. 3921/1981). Generally speaking, a reaction for thermal cracking of heavy petroleum oil induces a violent formation of bubbles at the beginning of the reaction due to cracked gas and cracked oil vapor. Since the reaction liquid is caused to flow upwardly in the above method, bubbles tend to occur in large volume in the lower part of the reactor, whereby requiring a column-like reactor of large volume. In addition, it is necessary to divide such reactor by means of a fairly large number of partition plates in order to render the quality of the resulting pitch suitable for the above-mentioned applications. This is because the reaction liquid is subjected to vigorous stirring by the rising flow of gas bubbles and the nearly through mixing with the gas.
This invention contemplates to provide an improved method for thermal cracking of heavy petroleum oil, which can achieve a high degree of cracking and, at the same time, produce as a byproduct a residual pitch rich in .beta.-resin components.
In one aspect of this invention, there is thus provided a method for the continuous thermal cracking of heavy petroleum oil which comprises continuously charging preheated heavy petroleum oil into the upper reaction zone of an upright cylindrical reactor, which is divided into an upper and a lower reaction zone by means of a partition plate defining an opening therethrough, so as to subject said preheated heavy petroleum oil to a thermal cracking reaction; drawing the resultant cracked gas and oil vapor from the upper reaction zone; causing the resultant reaction liquid to travel from the upper reaction zone through the lower reaction zone without back-mixing; allowing a small amount of cracked gas and cracked oil vapor, which have been formed in the course of the downward travelling of the resultant reaction liquid through the lower reaction zone, to rise through the lower reaction zone and be withdrawn from the reactor through the upper reaction zone; and discharging residual pitch through the bottom of the lower reaction zone.
Although the above method can certainly achieve its objects as far as its thermal cracking aspect is concerned, it still has some problems to be solved, including how the reaction solution should be cooled to terminate the cracking reaction.
Cooling a high temperature reaction liquid, in which a reaction is proceeding, so as to terminate the reaction, is generally effected by passing the reaction liquid through a cooling apparatus equipped with heat-exchanging surfaces and subjecting it to heat exchange with a cooling medium. Where a high temperature liquid is a substance having an extremely high viscosity and being susceptible of causing coking, such as a liquid under reaction in a thermal cracking reaction of heavy petroleum oil, a heat exchange cooling method is not practical because its heat-transfer coefficient is small and plugging tend to occur due to coking.
When pitch has been cooled down in accordance with such a cooling method as mentioned above, it contains not only gas bubbles dispersed and entrained therein but also gases dissolved therein and oils having low boiling points. When the thus-cooled pitch is pumped out of the cooling apparatus a vapor lock phenomenon is induced which renders its smooth discharge difficult.
In the second aspect of this invention, there is provided a method for continuous thermal cracking of heavy petroleum oil, which allows to conduct the cooling of the reaction liquid, in which the reaction has been brought to termination, without need for any heat-exchanging surfaces while at the same time obviating the occurrence of coking and also permitting an easy discharge of the resultant residual pitch by means of a pump.
As a continuous column-like thermal cracking apparatus a soaking drum of a visbreaker is used. In this case, the percentage of cracking is held low, i.e., 15-30% or lower for each raw material in order to keep the stability of the visbroken oil high. Thus, the progress of coking is extremely slow and, even if the soaking drum is provided with internal partition plates, it is not furnished with any scraper. The cleaning of the interior of the soaking drum is generally carried out periodically.
Where thermal cracking is effected with a longer residence time than that for cracking in a visbreaker and under such reaction conditions that a high degree of cracking is sought, a violent coke formation takes place on the inner wall of the reactor, whereby a long run operation of the reactor becomes impossible. Accordingly, in thermal cracking of heavy petroleum oil with a high degree of cracking, it is preferred to avoid the formation and sticking of coke on the inner wall and on the partition plates of the reactor.
In a third aspect of this invention, there is provided a method for continuous thermal cracking of heavy petroleum oil in which a preheated feed stream of heavy petroleum oil is introduced continuously into the upper reaction zone of a column-like reactor, cracked gas and oil vapor are drawn out of the reactor through an upper portion thereof and residual pitch is discharged through a bottom portion of the reactor. The method is characterized in that the reactor is provided with a rotary scraper having blades which are contiguous to their corresponding reactor walls or partition plates.